In US it's common that a house it's wired for both: the main electrical panel has the 3-wire 120/240 volts single-phase service coming in with two live wires. If you use one of them and the neutral, you get the 120V service (often called 115V service) and it's a three-conductor cable: ground (green), neutral (white) and live (black) . This is what most of the wiring in the house looks like.
Many high power appliances however (range, dryer, el. water heater etc.) , have four-conductor cables from the main panel that add a second live wire (red) and the voltage between the live wires is 240V (or 230V - it's the same thing and it's never exactly 230 or 240 anyhow). More energy can be delivered that way.
So, how many wires are in the cable coming from the panel to the pump? Assuming they are all connected, of course. If you only have one black live wire, then flip the switch to 115V setting. If you have black and red (and, again, both are hooked up), flip it to 230V.
- posted 8 years ago